A gist containing examples for copyWith code that can allow passing in null to a copy.

copy_with_examples.dart

void main() {
  print('Wont pass null into a copy');
  
  final e1 = Example1(id: 1, name: 'hello 1');
  final e1Copy = e1.copyWith(name: null);
  print('e1: ${e1.id}, ${e1.name}');
  print('e1Copy: ${e1Copy.id}, ${e1Copy.name}');
  
  print('\nSubclassing method');
  
  final e2 = Example2(id: 2, name: 'hello 2');
  final e2Copy = e2.copyWith(name: null);
  print('e2: ${e2.id}, ${e2.name}');
  print('e2Copy: ${e2Copy.id}, ${e2Copy.name}');
  
  print('\nCallable class method');
  
  final e4 = Example4(id: 4, name: 'hello 4');
  final e4Copy = e4.copyWith(name: null);
  print('e4: ${e4.id}, ${e4.name}');
  print('e4Copy: ${e4Copy.id}, ${e4Copy.name}');
  
}

// wont pass null into a copy

class Example1 {
  const Example1({
    required this.id,
    this.name,
  });
  final int id;
  final String? name;

  Example1 copyWith({
    int? id,
    String? name,
  }) {
    return Example1(
      id: id ?? this.id,
      name: name ?? this.name,
    );
  }
}

const SENTINAL = Object();

// subclassing method

abstract class Example2 {
  factory Example2({required int id, String? name}) = Example2Impl;
  const Example2._({
    required this.id,
    this.name,
  });
  final int id;
  final String? name;

  Example2 copyWith({
    int? id,
    String? name,
  });
}

class Example2Impl extends Example2 {
  Example2Impl({
    required super.id,
    super.name,
  }) : super._();

  @override
  Example2 copyWith({
    Object? id = null,
    Object? name = SENTINAL,
  }) {
    return Example2Impl(
      id: id == null ? this.id : id as int,
      name: name == SENTINAL ? this.name : name as String?,
    );
  }
}

// callable class method

class Example4 {
  const Example4({
    required this.id,
    this.name,
  });
  final int id;
  final String? name;

  Example4CopyWith get copyWith => Example4CopyWith(this);
}

abstract class Example4CopyWith {
  factory Example4CopyWith(Example4 value) = Example4CopyWithImpl;
  Example4 call({
    int? id,
    String? name,
  });
}

class Example4CopyWithImpl implements Example4CopyWith {
  const Example4CopyWithImpl(this._value);
  final Example4 _value;

  @override
  Example4 call({
    Object? id = null,
    Object? name = SENTINAL,
  }) {
    return Example4(
      id: id == null ? _value.id : id as int,
      name: name == SENTINAL ? _value.name : name as String?,
    );
  }
}